The present invention relates generally to fiber-reinforced composites wherein the matrix consists of a ceramic material. More particularly the invention relates to ceramic matrix composites comprising a matrix composed of an oxide glass or glass-ceramic together with reinforcing fibers composed of carbon, wherein the fibers are protectively coated with boron nitride to improve the properties of the composite products.
Proposed applications for ceramic matrix composites include components for heat engines. Such components must exhibit good strength and toughness at ambient and elevated temperatures in an oxidizing environment.
Fiber-reinforced ceramic matrix composites comprising glass and/or glass-ceramic matrices are well known. U.S. Pat. No. 4,626,515 for example, discloses fiber-reinforced composites wherein the reinforcing fibers are composed of silicon carbide and the reinforced matrix is an essentially alkali-free, alkaline earth aluminosilicate glass. U.S. Pat. Nos. 4,615,987, 4,589,900 and 4,755,489 disclose silicon carbide fiber reinforced glass-ceramic composites wherein the matrix materials consist of an alkaline earth aluminosilicate glass-ceramic composition.
Composites reinforced with silicon carbide fibers such as Nicalon.RTM. fibers excellent strength and toughness at high temperatures. However, these fibers are expensive, and in addition require relatively high composite processing temperatures, typically above 1300.degree. C., to develop the tough fracture behavior which is desired in the composite material. Graphite fibers, on the other hand, while relatively inexpensive, do not offer the high temperature performance characteristics of silicon carbide fibers, being most generally limited in utility to applications wherein composite use temperatures do not exceed about 500.degree. C.
One method of improving fiber performance in metal and ceramic matrix systems has been to apply coatings to the reinforcing fibers prior to incorporating them into the composite material. U.S. Pat. No. 4,397,901, for example, describes a composite article and method for making it wherein the fibers are provided with successive coatings of pyrolytic carbon, diffused silicon, and silicon carbide. U.S. Pat. No. 4,405,685 describes a similar coating system for carbon fibers wherein an inner coating consisting of a mixture of carbon and a selected metal carbide is used, while U.S. Pat. No. 4,642,271 describes the use of boron nitride coatings on silicon carbide or carbon fibers to be used to reinforce selected glass and ceramic matrix materials. U.S. Pat. Nos. 4,376,803 and 4,376,804 describe composite coatings consisting of amorphous carbon with overlayers of a metal oxide such as SiO.sub.2 for reinforcing metal alloys.
Notwithstanding the substantial efforts which have been devoted to the development of these coatings, no effective method for improving the high temperature functionality of graphite or other carbon fibers in glass and particularly glass-ceramic matrix materials has yet been developed. Yet a successful method for improving the high temperature performance of carbon fibers in ceramic composites systems could have a very significant commercial impact to the extent that decreases in cost and increases in the range of utility of these materials could be achieved.
It is therefore a principal object of the present invention to provide a method for protecting carbon fibers to be used for ceramic matrix reinforcement from the adverse effects of high temperature processing and use.
It is a further object of the invention to provide coated carbon fiber materials, including coated graphite fiber materials, offering improved high-temperature strengthening and/or toughening performance in ceramic matrix composite systems.
It is a further object of the invention to provide fiber-reinforced glass and glass-ceramic matrix composite products containing coated carbon reinforcing fibers offering improved composite properties at high temperatures.
Other objects and advantages of the invention will become apparent from the following description thereof.